1. Technical Field
The invention relates generally to epitaxial growth of semiconductor materials, and more specifically to methods for growing nitride-based film using varying pulses.
2. Related Art
Typically, nitride-based films (e.g., heterostructures) are grown using either Metal Organic Chemical Vapor Deposition (MOCVD), Molecular Beam Epitaxy (MBE), or Reactive Molecular-Beam Epitaxy (RMBE). MOCVD requires a relatively high growth temperature that makes it difficult to grow nitride-based films that include Indium (In) and/or Aluminum (Al). While MBE and RMBE allow growth at lower substrate temperatures, they are difficult to adopt for growing high quality insulating nitride films.
Current optoelectronic nitride devices require about twenty-eight percent Al to operate at wavelengths of approximately three hundred nanometers. For example, an optoelectronic nitride device can include an AlGaN (Aluminum-Gallium-Nitrogen) layer grown over a GaN buffer layer. Due to the lattice mismatch between the two layers, current deposition techniques cannot create a reliable AlGaN layer having a thickness greater than approximately one hundred fifty Angstroms. As a result, currently attainable thicknesses are insufficient for fabricating various semiconductor devices, such as a photodetector or a multiple quantum well light-emitting diode that operates at wavelengths of approximately three hundred nanometers.
Including In in one or both of the AlGaN and GaN layers has been shown to provide greater control over lattice mismatch and strain in the structure. The greater control allows for the creation of devices that are more reliable. A Pulsed Atomic Layer Epitaxy (PALE) technique was developed to deposit AlInGaN layers. PALE layer growth uses a series of metal organic precursor flux pulses to deposit the desired elements.
FIG. 1 shows an illustrative sequence of precursor flux pulses that generate an AlInGaN layer using the PALE technique. The sequence includes several series of pulses 10, 12, 14, 16. Series of pulses 10 applies Al using a precursor flux comprising Tri Methyl Aluminum (TMA), series of pulses 12 applies N using a precursor flux comprising NH3, series of pulses 14 applies In using a precursor flux comprising Tri Methyl Indium (TMI), and series of pulses 16 applies Ga using a precursor flux comprising Tri Methyl Gallium (TMG). Each series of pulses 10, 12, 14, 16 includes at least one pulse. In the sequence, only one precursor flux is pulsed at any given time 18. Each precursor flux flows at a constant rate during a pulse, and does not flow when not pulsed. As a result, each pulse is shown having a rectangular shape. The duration of each pulse was set at approximately six seconds. In the sequence, series of pulses 12 is pulsed every other time, with one of the metalorganic precursor fluxes 10, 14, 16 pulsing between each nitrogen precursor flux pulse.
While the PALE technique allows for the deposition of high quality epitaxial layers at temperatures close to those used by MBE and RMBE, it provides a relatively slow growth rate and does not incorporate any flexibility to allow for optimizing growth of the nitride-based films.
As a result, a need exists for improved methods of growing a nitride-based film that are more efficient and more flexible to produce higher quality, larger nitride-based films.